(1) Field of the Invention
The present invention relates to a multi-stage method for corrosion-protective and adhesion-promoting treatment of metal surfaces, encompassing a first method step for passivating pretreatment with an acidic aqueous composition (A) containing water-soluble compounds of Zr and/or Ti as well as fluoride ions, a subsequent method step for post-treatment with an aqueous composition (B) containing at least one organic compound having at least one aromatic heterocycle, the aromatic heterocycle comprising at least one nitrogen atom. The invention further relates to a metal surface treated in accordance with the method according to the present invention, and to the use of said treated metal surface for subsequent coating with an organic binding agent system.
(2) Description of Related Art
WO 2007/065645 discloses aqueous pretreatment solutions for corrosion-protective and adhesion-promoting conversion of metal surfaces prior to a subsequent electrodip coating process, which contain    a) no more than 1 mg/l organic polymer having allylamine or vinylamine monomers;    b) at least one further component that is selected from: nitrate ions, copper ions, silver ions, vanadium or vanadate ions, bismuth ions, magnesium ions, zinc ions, manganese ions, cobalt ions, nickel ions, tin ions, buffer systems for the pH range from 2.5 to 5.5, aromatic carboxylic acids having at least two groups that contain donor atoms, or derivatives of such carboxylic acids, silicic acid particles having an average particle size below 1 μm, and    c) a fluoro complex of at least one element M selected from the group B, Si, Ti, Zr, and Hf.
Aqueous compositions of this kind are suitable for corrosion-protective pretreatment, and possess the advantage as compared with conventional phosphating, for example in automobile manufacturing, that they can be used in methods that on the one hand comprise fewer treatment steps and on the other hand, in a context of continuous operation of a pretreatment line, have almost no tendency to form inorganic sludges that, in the case of phosphating, must be laboriously processed because of their heavy-metal content. Phosphating does, however, still possess definite advantages, in terms of adhesion to subsequently applied paint layers and in terms of the corrosion resistance of the crystalline phosphate layer especially on galvanized surfaces, as compared with an amorphous conversion layer based on mixed oxides and hydroxides of the metals Si, Ti, Zr, and Hf.
WO 2008/133047 discloses aqueous treatment solutions for the conversion of metal surfaces, containing fluoro complexes of the metals Ti, Zr, and Hf as well as organic compounds selected from arylamines, aminopolysaccharides, amino-modified phenols, and derivatives thereof, which can additionally contain ions of the elements Mg, Al, Zn, Cu, and Co. WO 2008/133047 further teaches an aqueous post-rinse that contains compounds selected from phosphoric acid, aminophenols, and organic phosphorus compounds. According to the invention, in the course of this post-treatment, specific layer weights in terms of the metallic and organic components on the metal surface are said to be present in a manner implemented for sufficient corrosion protection.